This invention relates to an electrical generator such as an alternator that is capable of providing current at two separate voltages.
A modern vehicle uses an alternator to power the vehicle's electrical system and to recharge a battery that provides standby electric power whenever the vehicle engine is not operating or when insufficient electric power is available from the alternator. The alternator includes a field winding, stator windings, and a rotating shaft that is driven through some arrangement by an engine. Rectifiers are used to convert the alternating current generated by the stator windings into direct current for battery charging and other electric loads. A voltage regulator senses the alternator output voltage and controls the field coil current to maintain a constant voltage according to the regulator's internal voltage reference as external electric loads are added and removed, within the limits of the alternator output power capacity. This is generally achieved by causing current to flow through the field winding whenever output voltage drops below the reference voltage, and stopping the flow of current through the field winding whenever the output voltage rises above the reference voltage.
The appropriate regulator reference voltage is determined by the battery charging voltage needed for the particular application, and the vehicle electrical system typically is designed to operate at this voltage. The reference voltage is often designed with temperature compensation because it is desirable for battery charging that the charging voltage decrease as battery temperature increases. Alternator output current is produced in the stator windings when the field winding is conducting current and the alternator shaft is turning.
Some vehicles that employ traction motors to drive the vehicle also use conventional automotive electrical systems for lighting and electronic systems that operate at either 14 volts or 28 volts. The electric power for the traction motors is typically derived from a main generator driven by an internal combustion engine. Battery power at 84 volts is typically used to crank the internal combustion engine and to activate the main generator field. During normal operation, electric power at either 14 volts or 28 volts is needed to power the automotive electrical system, and electric power at 84 volts is needed to keep the engine-cranking batteries fully charged.
Prior art dual voltage alternators often provide 14 volt and 28 volt output, because these two voltages are most commonly found in automotive electrical systems. These systems typically employ a common stator powered by a field coil to generate the output power for two voltages that share a common ground. As an example of a typical arrangement, the field coil is controlled in response to the 28 volt output only, with no rectifier control on the 28 volt supply, and the 14 volt supply is controlled via a switched rectifier such as a silicon controlled rectifier (SCR).
A potential disadvantage of this common stator arrangement is that output power at the higher voltage output (e.g. 28 volts) may not be available at low shaft speeds. This output power disparity at low shaft speeds may be acceptable if there is not a significant difference between the two output voltages, and if output power at both voltages is available at the lowest normal operating shaft speed. However, as the two output voltages diverge and the difference between them increases in magnitude (e.g. 28 volts-14 volts=14 volts, while 84 volts-28 volts=56 volts), output at the higher voltage may not be available except at a high alternator shaft speed. For example, an engine-driven common stator alternator operating at engine idle speed may have some 28 volt output current, but no 84 volt output current, unless the engine speed is significantly increased.
It is not apparent that anyone has addressed all of the above problems in an alternator or voltage regulator design. However, various systems have been proposed which touch upon some aspects of the above problems. For example, Mashing, et al., U.S. Pat. No. 4,788,486, proposes a vehicular power supply system that includes a field winding that generates a rotating magnetic field to induce alternating current (AC) voltages in a pair of sets of armature windings sharing a common ground. The AC voltages of the armature windings are converted by two groups of rectifiers to respective DC voltages that in turn charge a pair of batteries in series. A first voltage regulator controls the current of the field winding to regulate the first battery voltage. A second voltage regulator regulates the second battery voltage by connecting and disconnecting the second battery from a group of rectifiers. Mashing does not disclose independent switching or control of the groups of rectifiers. Neither the second voltage regulator nor the second battery appears to have any effect on the field winding, which is initially excited and is thereafter self-excited and modulated according to the value of the first battery.
Abukawa, et al., U.S. Pat. No. 5,033,565, proposes a generator that generates two voltage outputs. A field winding, responsive to a predetermined exciting current supplied from a voltage regulator, induces three-phase AC voltages in a pair of armature windings. First and second DC voltages are generated at a pair of output terminals from the AC voltages by two groups of rectifiers. Abukawa, et al., does not consider voltage regulation schemes beyond supplying a predetermined exciting current. Neither group of rectifiers is controlled by the voltage regulator, which is not illustrated. The armature windings are shown to be in mechanically close proximity around a drive shaft in FIG. 2 of Abukawa, and appear to be of the common ground variety. The DC output voltages appear to be commonly grounded in all pictorial embodiments of the generator.
Baumgartner, et al., U.S. Pat. No. 5,033,565, proposes a generator that employs a pair of identically designed stators wound in mechanically close proximity to attempt to generate two identical voltage outputs. A field winding supplies the alternator field. A generally conventional voltage regulator maintains the proper excitation voltage across the field winding at engine speed above low idle for AC outputs from the stators that will provide DC outputs that are as equivalent to each other as possible in response to balance and unbalanced loads. It appears to be a design goal that the DC voltage outputs be maintained essentially identical in magnitude, and that the stators be identical in size and function. The voltage regulator controls neither group of rectifiers.